The present invention relates to a printing hammer device which is adapted to be mounted on a printing apparatus, such as an impact printer, and performs a printing operation by striking, in a printing position, character elements arranged on a daisy wheel or the like.
In conventional printing hammer devices, such as the one disclosed in U.S. Pat. No. 4,401,026, front and rear mounting members, a coil bobbin, a resilient member, and a pair of yoke members are mounted in a case. The bearing members movably support striking and nonstriking-side end portions of a printing hammer. The coil bobbin is wound with a coil for magnetically driving the printing hammer, and has a center hole penetrated by the hammer. The resilient member is formed of a coil spring which causes the magnetically driven hammer to return to its initial position. The yoke members form a magnetic path which serves for the magnetic drive of the hammer. If the coil is supplied with an exciting current to produce a desired impact, corresponding to the printing-surface area of the character element situated in the printing position, the printing hammer is moved toward the character element, against the resilience of the resilient member, by a magnetic field which is generated in the magnetic path between the case, the yoke members, and the hammer. Thus, the character element is struck with the desired impact for printing.
According to the prior art printing hammer devices described above, however, it is difficult to maintain high concentricity in mounting the printing hammer, bearing members, yoke members, and coil bobbin in the case, due to the use of a good many components. Accordingly, the sliding resistance of the printing hammer, against the other components, is uneven, so that the character elements cannot be struck with the desired impact, thus lowering the print quality. Moreover, a number of components must be arranged in the case with high concentricity, so that the device inevitably is large-sized, and cannot be assembled with high efficiency.
If the mounting accuracy of the components is not high enough, the striking portion of the printing hammer cannot accurately face the character element in the printing position. In such a case, the character element may possibly be struck together with its adjoining one, thereby causing misprints. Therefore, the printing hammer device must be mounted with high positional accuracy, on a traveling member, such as a carriage, carrying a printing head. Thus, the mounting work requires much time, thereby further lowering the assembling efficiency.
In order to stabilize and increase the speed of the operation of the printing hammer, in the printing hammer devices of this type, rebounding of the hammer must be minimized, which may be caused when the hammer, moving to its initial position or rest position after the striking action, abuts against a hammer socket.
To cope with this, in the prior art printing hammer devices, the hammer socket, which moves in engagement with the nonstriking-side end of the printing hammer, returning toward its initial position by means of the resilience of the resilient member, is increased in mass. By doing this, the returning energy of the printing hammer can be absorbed by means of the hammer socket itself, as well as a shock absorbing member, which is subjected to elastic deformation as the hammer socket moves.
Disclosed in U.S. Pat. No. 4,272,748 is an arrangement including a print wire in which a precompressed elastomer material, is used as the shock absorbing member.
According to the conventional printing hammer devices described above, however, when the hammer socket is caused to return to its initial position, by the returning energy of the printing hammer, not absorbed by the shock absorbing member yet, as well as by the resilience of the shock absorbing member itself, the printing hammer, still subjected to a considerable reaction force from the hammer socket, cannot help moving again toward the character element. Thus, the hammer cannot be prevented effectively from rebounding. Therefore, it takes much time for the hammer to be settled in its initial position, so that the printing speed cannot be increased. If the printing hammer is operated without being stabilized, the impact on the character element cannot be uniform, thus failing to provide high-quality prints.